By utilising both advantages of the UAV and the RIS, the received signal at the ground users is strengthened while the power consumption is reduced and the flying time of the UAV can be extended [1]}, [2]}, [3]}, [4]}. In [2]}, the UAV's trajectory and the RIS's passive beamforming vector were optimized to maximize the average rate in RIS-assisted UAV communications. The problem was derived into two subproblems; then, a closed-form phase shift algorithm was introduced to find the local optimal reflective matrix and the successive convex approximation was used to find the suboptimal trajectory solution. In [3]}, the UAV acts as a mobile relay and the RIS was used to provide short packets communications ultra-reliable and low-latency between ground transmitter and ground IoT devices. The UAV's position, the RIS phase shift and the blocklength were optimized to minimize the total decoding error rate by using a polytope-based method, namely Nelder-Mead simplex.
